The weakest base among the following is

Options

A. NaOH
B. KOH
C. `Ca(OH)_2`
D. `Zn(OH)_2`

Explanation

To determine the weakest base among the given options, we need to analyze the bases provided: NaOH, KOH, Ca(OH)₂, and Zn(OH)₂.

### Step-by-Step Solution:

1. **Identify the Bases**: The bases given are:
- Sodium hydroxide (NaOH)
- Potassium hydroxide (KOH)
- Calcium hydroxide (Ca(OH)₂)
- Zinc hydroxide (Zn(OH)₂)

2. **Understand Basicity**: Basicity refers to the ability of a compound to donate hydroxide ions (OH⁻) in solution. The stronger the base, the more readily it dissociates to release OH⁻ ions.

3. **Group Classification**:
- NaOH and KOH are alkali metal hydroxides (Group 1A).
- Ca(OH)₂ is an alkaline earth metal hydroxide (Group 2A).
- Zn(OH)₂ is a hydroxide of a transition metal.

4. **Comparing Group 1A and Group 2A Hydroxides**:
- Generally, alkali metal hydroxides (Group 1A) are stronger bases than alkaline earth metal hydroxides (Group 2A) due to the larger size of the metal ions which leads to longer bond lengths and weaker M-OH bonds.
- Therefore, NaOH and KOH are stronger bases compared to Ca(OH)₂.

5. **Comparing Calcium Hydroxide and Zinc Hydroxide**:
- Calcium (Ca) is larger than Zinc (Zn), which means the bond length in Ca(OH)₂ is longer than in Zn(OH)₂.
- A longer bond length in Ca(OH)₂ indicates a weaker bond and thus a stronger base compared to Zn(OH)₂.
- Zinc hydroxide (Zn(OH)₂) is less soluble in water and does not dissociate as readily, making it a weaker base.

6. **Conclusion**: Among the given options, Zn(OH)₂ is the weakest base due to its lower solubility and weaker dissociation compared to the other hydroxides.

### Final Answer:
The weakest base among the following is **Zn(OH)₂**.